1. Field of the Invention
The present invention relates to a semiconductor device used for a switching device for carrying out on-off control of a current flowing in an electric circuit and a method of detecting a wire open failure of the semiconductor device. The invention particularly relates to a semiconductor device controlling a large current like an ignition semiconductor device for an automobile internal combustion engine.
2. Background Art
For an ignition system for an automobile internal combustion engine, a power semiconductor device is used which carries out, for example, switching control of a primary current of an ignition coil. FIG. 9 is a circuit diagram showing an example of a configuration of a common ignition system for an automobile internal combustion engine using an IGBT (Insulated Gate Bipolar Transistor) for such a power semiconductor device.
The ignition system for automobile internal combustion engine shown in FIG. 9 is formed of an engine control unit (hereinafter referred to as an ECU) 901, an ignition IGBT 902 as a related ignition IGBT, an ignition coil unit 903, a voltage source 904 and a spark plug 905.
The related ignition IGBT 902 has three electrodes of a collector electrode C connected to the ignition coil unit 903, an emitter electrode E connected to the ground, and a gate electrode G connected to the ECU 901. Between the gate electrode G and the emitter electrode E, a bidirectional Zener diode 908 is connected that protects the gate of an IGBT 909 in case a surge voltage exceeding the breakdown voltage of the Zener diode is inputted to the gate electrode G.
Here, an operation of the ignition system for automobile internal combustion engine shown in FIG. 9 will be briefly explained. The ECU 901 outputs a signal to the gate electrode G for controlling the switching (turning-on and -off) of the ignition IGBT 902. For example, a signal of 5V outputted to the gate electrode G brings the IGBT 909 to be turned-on and a signal of 0V outputted to the gate electrode G brings the IGBT 909 to be turned-off.
First, a turning-on signal outputted from the ECU 901 to the G terminal brings the ignition IGBT 902 to be turned-on, which allows a corrector current (hereinafter referred to as an Ic) to begin to flow from the voltage source 904 (at 14V, for example) to the collector electrode C of the ignition IGBT 902 through a primary coil 906 of the ignition coil unit 903. A rate of change of the Ic with respect to time t expressed as dIc/dt is determined by the inductance of the primary coil 906 and an applied voltage. Subsequent to this, a turning-off signal outputted from the ECU 901 to the G terminal makes the ignition IGBT 902 turned-off to abruptly decrease the Ic. The abrupt change in the Ic abruptly increases the voltage across the primary coil 906. At the same time, the voltage across a secondary coil 907 also increases up to tens of kilovolts (30 kV, for example). The increased voltage is applied to the spark plug 905. The spark plug 905 produces a spark at approximately 10 kv or more of an applied voltage.
The related ignition IGBT 902, on which previously explained switching control is carried out, will be explained with reference to FIGS. 14A and 14B. FIG. 14A is a circuit diagram again showing the configuration of the related ignition IGBT 902 shown in FIG. 9 and FIG. 14B is a plan view showing the related ignition IGBT 902 with resin for encapsulation being removed therefrom. The related ignition IGBT 902 has a structure in which an IGBT chip 1001 is soldered on a C terminal board 1000-1(a) of a lead frame 1000 with the collector electrode C on the bottom surface side down. The gate electrode G on the top surface side of the IGBT chip 1001 is connected to an external G terminal board 1000-2 by bonding with a metal wire 1003 and the emitter electrode E on the top surface side of the IGBT chip 1001 is connected to an external E terminal board 1000-3 by bonding with another metal wire 1003. A bidirectional Zener diode ZD, connected between the gate electrode G and the emitter electrode E of the IGBT chip 1001, is formed in a polysilicon layer layered on the surface of the IGBT chip 1001 with an oxide film in between. From one side of the C terminal board 1000-1(a) forming the lead frame 1000, an external C terminal board 1000-1(b) is extended for connecting the C terminal board 1000-1(a) to the outside. Furthermore, the IGBT chip 1001 and the wire bonded part at one end of each of the external terminal boards are sealed so as to be covered with unillustrated molding resin. The other end of each of the external terminal boards protrudes outside of the molding resin to be exposed.
When a large current flows in an IGBT like the ignition IGBT 902, the IGBT chip 1001 generates heat due to on-state resistance. Moreover, the metal wire 1003, connecting the surface of the emitter electrode E of the IGBT chip 1001 and the surface of the external E terminal board 1000-3, generates heat due to the electric resistance thereof.
Moreover, as the electric resistance of the metal wire 1003 becomes larger, a power loss due to the current flowing in the metal wire 1003 becomes larger. In addition, as the inductance of the metal wire 1003 becomes larger, the metal wire 1003 becomes more liable to cause an oscillation. Hence, it is desirable for the metal wire 1003 to have the smallest possible electric resistance and inductance. In general, the electric resistance of a metal wire can be decreased by increasing the diameter of the metal wire. However, there is a limit in the diameter of a metal wire that allows the metal wire to be reliably connected by bonding. Thus, it is impossible to further decrease the electric resistance of the metal wire by increasing the diameter of the metal wire more than the limit.
Therefore, even in an actual related ignition IGBT, a connection structure, in which an emitter electrode of an IGBT chip and an external E terminal board are connected with a plurality of metal wires arranged in parallel by bonding, is commonly adopted for minimizing heat generating factors such as electric resistance, thermal resistance and inductance when a current is made to flow in the metal wire. FIG. 13A is a plan view showing a related ignition IGBT similar to the related ignition IGBT shown in FIG. 14B with resin for encapsulation being removed therefrom, in which the emitter electrode E of the IGBT chip 1001 and the external E terminal board 1000-3 are connected by a single metal wire 1003. FIG. 13B is a plan view showing a related ignition IGBT with resin for encapsulation being removed therefrom in which the emitter electrode E and the external E terminal board 1000-3 of the IGBT chip 1001 shown in FIG. 13A are connected by a plurality of metal wires 1003 arranged in parallel.
The ignition IGBT with such a connection structure of the metal wires passes an early stage electrical factory test carried out prior to shipment even though a part of a plurality of the metal wires 1003 are opened (by breaking of wire, for example) and is to be put on the market. However, breaking in a part of a plurality of the bonded metal wires in parallel connection causes an increase in a current density per one metal wire to increase heat generation, by which temperatures of the metal wire itself, a section of connecting the metal wire and the semiconductor chip by bonding and the semiconductor chip itself become high to increase possibilities of causing problems in durability and long-term reliability. A product of a device, having such a high possibility of causing deterioration or degradation in durability and reliability thereof even though it has passed an early stage electrical factory test carried out prior to shipment, is unfavorable as a product in a field where a requirement for high reliability is particularly severe like a vehicle-mounted component. Hence, such a product of a device must be reliably sorted out as a faulty component in an electrical factory test at an early stage. The connection structure, having wire open failures with a part of a plurality of metal wires 1003 opened as was explained in the foregoing, is considered to be caused by defects such as faulty solderless connection at bonding and breaking due to a mechanical or electrical cause.
With respect to a method of reliably detecting wire open failure in a power semiconductor device, having a structure in which a plurality of metal wires arranged in parallel are connected by bonding, in an early stage electrical factory test carried out prior to shipment, documents with descriptions like those in the following have been already known publicly. For example, in JP-A-9-266226, it is described that in a semiconductor device with a number of unit cells arranged in parallel on a semiconductor substrate, a configuration is provided in which at least one of the main electrode regions of the semiconductor device is split into two or more individual bonding pad regions and, to a bonding pad in each of the regions, an individual bonding wire is connected with one end thereof, and the other end of the bonding wire is connected to a common external terminal. By measuring the on-state resistance of the semiconductor device, wire open failure of the device can be detected.
Moreover, in JP-A-11-111785, a wire open detecting system is described in which a plurality of first pads, being in correspondence with a plurality of their respective device cells, are connected to a single first lead (terminal) with a plurality of wires the number of which is the same as that of the first pads, the first pads are further connected to a single second pad for the wire open failure detection through their respective resistors, and the second pad is connected to a single second lead (terminal) for the wire open detection with a wire. In the wire open detecting system, wire open failure can be detected by a change in the resistance value between the first lead and the second lead.
Patent Document 1: JP-A-9-266226
Patent Document 2: JP-A-11-111785
The method disclosed in JP-A-9-266226 is a method of detecting a wire open failure by measuring the on-state resistance of a device and the method disclosed in JP-A-11-111785 is a method of detecting a wire open failure by measuring the variation in the resistance value between the single first lead (terminal), to which a plurality of device cells are connected through their respective wires, and the single second lead (terminal), to which the wires are connected through their respective resistors.
The method of detecting a wire open failure disclosed in each of JP-A-9-266226 and JP-A-11-111785 detects wires with open failures by the change in resistance value depending on the number of wires with open failures. Thus, although the methods can detect the number of wires with open failures, it is difficult for the methods to identify the wire with an open failure. Furthermore, the amount of change in resistance value due to an open failure decreases with an increase in the number of wires. This causes the change in resistance value due to the wire open failure to become equal to or less than the range of variation in resistance values among the semiconductor devices, which sometimes makes the detection of open failure difficult.
The invention was made with the problems explained in the foregoing taken into consideration. It is an object of the invention to provide a semiconductor device which is capable of detecting an open failure of a bonding wire regardless of the number of a plurality of the bonding wires connected in parallel by a simple electrical test to make it possible to reliably sort out a semiconductor device with a wire open failure at an early stage, and a method of detecting a wire open failure.